Lattice diffusion coefficient, high-temperature

Solid-State Diffusion Coefficient 343 Temperature Dependence 343 Values of D for Lattice Diffusion 345 High Diffitsivify Paths 346... 

According to the analysis in the previous sections, the primary particle size in flame reactors is determined by the relative rates of particle collision and coalescence. For highly refractory materials, the characterislic coalescence time (12.6) depends on the solid-state diffusion coefficient, which is a very sensitive function of the temperature. The mechanisms of solid-.staie diffusion depend in a complex way on the structure of the solid. For example, a perfect cubic crystal of the substance AB consists of alternating ions A and B. Normally there are many defects in the lattice structure even in a chemically pure single crystal defect types are shown schematically in Fig. 12.8. The mechanism of diffusion in cry.stalline solids depends on the nature of the lattice defects. Three mechanisms predominate in ionic... 

The summary of the structural, thermodynamic and transport properties determined by various authors (as discussed above) has been shown in Tables 12.3,12.4 and 12.5, respectively. The lattice constant values calculated by different authors for GaN have been observed to be in good agreanent. The same has been found to be true for the AIN alloy also. However, the room temperature linear thermal expansion coefficient values for GaN vary widely. Unlike GaN, the expansivities for the AIN alloy, as determined by different studies, are in good agreement and the values for the other binary alloys are of similar order. The isochoric heat capacity, Cy, values show that these alloys follow the Dulong-Petit law for solids whereby at high tanperatures, C 3/ . It should also be noted that the diffusion coefficients for the binary nitrides have similar values at room temperature. 

When atoms displace from one state to another one in lattice, which has to pass the mediacy of high energy, the necessary energy is called as activation energy of the process. The temperature relationship of the diffusion coefficient can be expressed as the following equation ... 

Experiments were performed to determine the effect of structure on the mechanism of zirconium oxidation. It is shown that there is a high volume fraction of short circuit diffusion paths in the oxide formed on Zr in the temperature range 500-1000°C. The results of tracer experiments are in agreement with a short circuit diffusion mechanism. The assumption of line diffusion is shown to imply intuitively reasonable values of both the grain boundary thickness and the ratio of the line diffusion coefficient to the coefficient for lattice diffusion. 

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